1. Field of the Invention
The invention relates generally to the determination of orientation through sensing Earth's magnetic field, and more particularly to a method and apparatus to correct compass magnetic heading and/or position sensing indications to account for motion and/or magnetic distortions.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Orientation instrumentation often uses acceleration or tilt sensors to determine the tilt angle relative to earth horizontal reference. Most of these tilt sensors are sensitive to motion acceleration, distinct from the acceleration of gravity, which can result in large orientation errors during periods of movement. The orientation error is directly seen in pitch and roll errors, as well as indirectly seen in yaw errors.
One means of compensating for motion induced orientation errors which has been previously employed is the use of the gyroscopic effect to monitor rotation rates and correct for the motion induced errors, since gyros are not generally subject to the same errors. This method has the disadvantage in practice that accurate and stable gyros tend to be large, expensive, and have relatively large power requirements. Inexpensive gyros tend to have high drift rates, limiting their usefulness to short duration motions. In either case, gyros add additional complexity to an orientation system.
Previous patents cover various methods of integrating gyros into compassing and orientation applications, such as U.S. Pat. No. 6,842,991 by Robert Levi and Robert Marshall, issued 18 Jan. 2005. Clymer Technologies disclosed U.S. 2004/0017192, now abandoned, as an orientation sensor with four magnetic sensors and a method for calculating magnetic inclination. U.S. Pat. No. 6,842,991 and U.S. 2004/0017192 are incorporated by reference herein in their entirety.
Clymer Technologies' Terrella6™ offers a heading and motion sensor capable of determining roll, pitch, yaw, translational accelerations, angular rates, and compass heading. The sealed device contains no moving parts and has interference detection to reduce ferrous metal disturbances.
The method disclosed by the instant invention makes use of an ambient magnetic field as a rotation reference during periods of motion.